Products containing light emitting diodes (LEDs) or lasers that emit light in the visible or infrared range are required to conform to eye safety requirements in the International Electrotechnical Commission (IEC) standard 60825-1. If the optical flux that can impinge on a user's eye exceeds the standard set out in IEC 60825-1, the device must be labeled an eye safety hazard. This requirement applies not only during normal operation of the circuit, but also when predictable single faults occur in the circuit.
Fuses can be used in a safety circuit for lasers. When the bias current to a laser light emitter exceeds the eye safety requirement, the fuse blows. Alternatively, a retriggerable transistor circuit can be used that shunts current away from a laser light emitter when the current exceeds a design threshold.
Alternatively, a monitoring photodiode having an output indicative of the actual output of the laser can be used. For example, a comparator compares a reference signal indicative of a reference output level of a laser and a monitor photodiode signal. The comparator output signal reflects the change in the actual optical output level of the laser. An abnormal current eliminator controls the comparator output so that the signal has limited amplitude. Hence, the drive current to the laser is forcibly decreased to safe drive current levels. See U.S. Pat. No. 4,884,280 issued to Kinoshita for “Semiconductor Laser Driving Device for Stabilizing the Optical Output Thereof”.